溶胶—凝胶法尺寸选择性合成ZnO纳米颗粒及其光催化性能

袁慧敏+吕林林+李亚萍+钱东

摘要在溶胶凝胶合成过程中，分别利用超声波处理ZnO溶胶、加水至溶胶中煮沸、加庚烷至溶胶中、蒸发除去部分溶剂等简单的沉淀方法制备了10 nm以下不同尺寸的ZnO颗粒.在60 ℃下通过超声波处理ZnO溶胶可得到高质量的ZnO纳米颗粒，颗粒粒径为6.2±1.5 nm，标准偏差为8%.同时，基于作者以前的发现，提出了在该溶胶凝胶法中新的ZnO形成机理.在光催化降解甲基橙过程中，纳米ZnO不同的颗粒尺寸及团聚情况导致其光催化活性有所差异，具有较小粒径和团聚较轻的ZnO纳米颗粒显示出较高的光催化活性.

关键词ZnO纳米颗粒；沉淀；溶胶凝胶法；颗粒尺寸；分散性；光催化

The properties of ZnO nanocrystals are highly dependent on their sizes and agglomeration situations. The synthesis of ZnO nanocrystals with controllable sizes and less agglomerations is still one of the most challenging and urgent topics.

It is well known that a facile precipitation method is an operative way to synthesize ZnO nanocrystals with different sizes. Hoyer et al[11] precipitated ZnO nanoparticles through the addition of water to the boiling colloidal ZnO solution. Meulenkamp[2] achieved this by using alkanes， e.g.， heptane， to precipitate ZnO nanoparticles. In our previous paper[12]， we found that sonicating the colloidal ZnO solution could also precipitate ZnO nanoparticles without using any solvents. Inevitably， these different methods for the precipitations of ZnO nanoparticles have effects on the sizes， polydispersities and agglomerations of ZnO nanoparticles.

Herein， we prepared ZnO nanoparticles with different sizes by diverse facile precipitation methods during a solgel synthesis procedure and investigated the influences of these precipitation methods on the sizes， polydispersities and agglomerations of ZnO nanoparticles. Meanwhile， their photocatalytic properties were also studied and a new mechanism for the formation of ZnO nanocrystals via the solgel route was proposed based on our previous findings.

1Experimental

1.1Syntheses of colloidal ZnO solution

The synthesis of colloidal ZnO solution was similar to the methods described by Meulenkamp[2] and in our previous paper[13]. A 2.86 g （13 mmol） amount of Zn（Ac）2·2H2O was placed in 130 mL of absolute ethanol， and then the mixture was heated to dissolve Zn（Ac）2·2H2O under magnetic stirring. When Zn（Ac）2·2H2O was dissolved completely， the Zn2+containing solution obtained was diluted to 130 mL by the addition of absolute ethanol and cooled to 0 ℃. 0.754 g of LiOH·H2O （18 mmol） was dissolved in 130 mL of absolute ethanol at room temperature under magnetic stirring. The hydroxidecontaining solution was then added dropwise into the Zn2+containing solution at 0 ℃ under stirring. The sol obtained was the mixture of ZnO colloid and an intermediate of hydroxy double salt Zn5（OH）8（Ac）2·2H2O （ZnHDS）. The hydroxy double salt could easily transform into ZnO phase through sonicating or heating[13].

1.2Precipitation of ZnO nanoparticles

Different methods were employed to treat the colloidal ZnO solution obtained until white precipitates were formed： （1） sonicating the colloidal ZnO solution by an ultrasonic cleaner at 60 ℃ to give sample A， （2） adding 1 mL of water to 50 mL of the colloidal ZnO solution and then boiling it to result in sample B， （3） sonicating the colloidal ZnO solution for 20 min at 0 ℃ to ensure the transformation of the hydroxy double salt intermediate into ZnO phase and then adding heptane to it to lead to sample C， and （4） removing part of the solvent by distillation to produce sample D.

1.3Characterization of Xray powder diffraction （XRD） and transmission electron microscopy （TEM）

The XRD patterns of the asprecipitated ZnO nanoparticles were characterized by a RigakuDMax rA 12 kW diffractometer with CuKα radiation （λ=1.54056 ） at an operation voltage and current of 40 kV and 300 mA， respectively. TEM measurements were performed on a JEM2010 microscope operated at an acceleration voltage of 200 kV.

1.4Photocatalytical degradation of methyl orange

A 0.20 g amount of the sample A， B， C or D was put into 150 mL of methyl orange solution with a concentration of 20 mg/L. Prior to the irradiation， the suspension was sonicated for 20 min， magnetically stirred in a dark condition for 30 min to establish an adsorption/desorption equilibrium， and then put into a selfassembly reactor. The irradiation resource of the photocatalytic reactor was four 40 W UV lamps set in parallel from the suspension 〖HJ2mm〗surface of 20 cm with a maximum emission at ca. 365 nm. Samples were taken from the reaction suspension at a 20 min interval. The upper lucid liquid obtained after centrifugal separation was analyzed by a UV─Vis spectroscopy.

2Results and discussion

2.1XRD analysis of samples

〖TPS97.TIF；S；Z2；Y1，Y#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.1〖STBZ〗〖WTBZ〗〖ZK（〗XRD patterns of the asprepared samples AD〖ZK）〗

〖TS）〗

The XRD patterns of the asprepared samples are shown in Fig.1. The reflections recorded can be indexed to hexagonal ZnO （JCPDS 361451）. The peaks are overlapped， which are caused by linebroadening because of the small crystal size. Average crystal sizes for the samples A， B， C and D， estimated by XRD using the Scherrer formula for the （102） reflection， are 6.5， 7.7， 4.7 and 6.4 nm， respectively.

〖STHZ〗〖WTHZ〗2.2Size distribution and agglomeration situation of samples〖ST〗〖WT〗

The TEM images and corresponding size histograms of the samples A， B， C and D are presented in Figs. 25， respectively， and the size distributions analyzed from the normal curves and agglomeration situations are listed in Tab.1.

〖DZ（85mm，85mmK0〗

〖TPS98.TIF；S*2，BP#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.2〖STBZ〗〖WTBZ〗〖ZK（〗TEM image and particle size distribution for the sample A precipitated by sonicating the colloidal ZnO solution〖ZK）〗

〖TS）〗

〖TPS99.TIF；S+3.5mm，BP#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.3〖STBZ〗〖WTBZ〗〖ZK（〗TEM image and particle size distribution for the sample B precipitated by adding water to the colloidal ZnO solution and then boiling it〖ZK）〗

〖TS）〗

〖DZ）〗

〖DZ（85mm，85mmK0〗

〖TPS100.TIF；S*2，BP#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.4〖STBZ〗〖WTBZ〗〖ZK（〗TEM image and particle size distribution for the sample C precipitated by the addition of heptane to the colloidal ZnO solution〖ZK）〗

〖TS）〗

〖TPS101.TIF；S+2.5mm，BP#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.5〖STBZ〗〖WTBZ〗〖ZK（〗TEM image and particle size distribution for the sample D precipitated by evaporating part solvent of the colloidal ZnO solution〖ZK）〗

〖TS）〗

〖DZ）〗

〖KH-1D〗

〖HJ1.5mm〗〖JZ〗〖WT5"HZ〗〖STHZ〗Tab.1 Size distributions and agglomeration situations of samples A， B， C and D analyzed from their TEM images〖HT5"SS〗

〖BG（！〗

〖BHDFG4.5mm，WK30mm，WK40mm，WK50mm，WKW〗

Samplediameter/nmstandard deviation/%agglomeration situation

〖BHD〗A6.2±1.58less

〖BHDW〗B7.4±2.613serious

〖BH〗C4.6±1.512medium

〖BH〗D6.6±1.910medium

〖BG）F〗

〖HJ1.55mm〗

Mean diameters of the samples A， B， C and D determined by TEM are 6.2， 7.4， 4.6 and 6.6 nm， respectively， which are in good agreement with the average crystal sizes estimated by XRD. It is well known that the sonochemical synthesis has become a routine method for preparing a wide variety of nanostructured materials[12，14]， which is based on the acoustic cavitation resulting from the continuous formation， growth and implosive collapse of bubbles in a liquid. The sample A was precipitated by sonicating the colloidal ZnO solution at 60 ℃， and the ZnO nanoparticles obtained had a rather narrow size distribution with a particle diameter of 6.2±1.5 nm， standard deviation of about 8%， and less agglomeration. The sample B was precipitated through adding 1 mL of water to 50 mL of the colloidal ZnO solution and then boiling it， which gave a particle size of 7.4±2.6 nm， standard deviation of ca. 13%， and serious agglomeration. This may be because that water and heating could accelerate the particle growth， and water could increase the particle agglomeration， which are also confirmed by the samples C and D. The sample C， which was precipitated by adding heptane to the colloidal ZnO solution at 0 ℃， had a particle size of 4.6±1.5 nm， standard deviation of around 12%， and medium agglomeration. However， the TEM image of the sample C is blurred， which could be attributed to the less crystallinity resulting from the low temperature treatment of the colloidal ZnO solution. The polydispersity， mean diameter and agglomeration degree of the sample D decreased in comparison with the sample B probably due to the fact that no water was added to the colloidal ZnO solution produced during the solgel synthesis of ZnO nanoparticles.

2.3Mechanism for the formation of ZnO nanocrystals

There are some controversies about the mechanism for the formation of ZnO nanocrystals during the solgel synthesis[13]. In Spanhel and Andersons procedure for the preparation of ZnO， they mentioned an organometallic Zn precursor， containing acetic acid derivatives， produced by refluxing an ethanolic Zn（Ac）2·2H2O solution before the addition of LiOH·H2O [1]. Later， Spanhel et al [15] attributed the organometallic Zn precursor to Zn10O4（Ac）12. However， the precursor was identified to be Zn4O（Ac）6 by Briois et al [16]. In our previous publications[1213]， we reported that a hydroxy double salt Zn5（OH）8（Ac）2·2H2O （ZnHDS） intermediate is formed and could directly transform into a ZnO phase in an acetatecontaining solution during the present solgel synthesis of ZnO nanocrystals described above. Therefore， the mechanism for the formation of ZnO nanocrystals in the acetatecontaining solution could roughly be described as：

4Zn（Ac）2 + H2O → Zn4O（Ac）6 + 2HAc〖JY〗（1）

5Zn4O（Ac）6 + 22LiOH + 5H2O→ 4Zn5（OH）8（Ac）2 + 22LiAc〖JY〗（2）

Zn5（OH）8（Ac）2 + 2LiOH5ZnO + 2LiAc + 5H2O〖JY〗（3）

HAc + LiOH → LiAc + H2O〖JY〗（4）

In Eq.1 the precursor with probable formula of Zn4O（Ac）6 is formed by the prehydrolysis of the ethanolic Zn（Ac）2·2H2O solution. The ZnHDS intermediate is present after the addition of LiOH·H2O into the ethanolic Zn4O（Ac）6 precursor solution in Eq.2. In Eq.3， the ZnHDS intermediate transforms into ZnO particles by further hydrolysis and the ZnO phase can also transform back to the ZnHDS phase through the dissolution/reprecipitation of ZnO nanoparticles， and a neutralization reaction between acetic acid （HAc） and LiOH exists in Eq.4.

2.4Photocatalytic activities of samples

〖TPS102.TIF；%115%115；S*2；Z1，Y#〗〖TS（〗

〖WT6HZ〗〖STHZ〗Fig.6〖STBZ〗〖WTBZ〗〖ZK（〗Photocatalytic degradation of methyl orange in the presence of the samples AD〖ZK）〗

〖TS）〗

Fig.6 exhibits the photocatalytic degradation of methyl orange in the presence of the samples A， B， C and D. After irradiating for 120 min， the degradation rates of methyl orange are 896%， 64.6%， 91.1% and 78.0% for the samples A， B， C and D， respectively. The photocatalytic activities of samples A and C are comparable， being better than the sample D， while the sample B is the worst. The samples A and C have higher photocatalytic activities may be due to their smaller particle sizes and less agglomerations， while the introduction of water and boiling for the sample B leading to the particle growth and serious agglomeration may account for the lowest photocatalytic activity.

〖HJ1.7mm〗

3Conclusions

〖KH-1〗

ZnO nanoparticles with different sizes less than 10 nm have been successfully synthesized by diverse facile precipitation methods via a solgel route. Different precipitation methods have evident effects on the sizes， polydispersities and agglomerations of ZnO nanoparticles. ZnO nanoparticles， precipitated by sonicating the colloidal ZnO solution at 60 ℃ （sample A）， adding water to the colloidal ZnO solution and then boiling it （sample B）， adding heptane to the colloidal ZnO solution （sample C）， and evaporating part solvent of the colloidal ZnO solution （sample D）， have particle diameters of 6.2±1.5， 7.4±2.6， 4.6±1.5 and 6.6±1.9 nm with standard deviations of about 8%， 13%， 12% and 10%， respectively. The agglomeration situation for the sample A is the least， and that for the sample B is the most serious. In the photocatalytic degradation of methyl orange， the samples A and C have the comparably better activities， which can be attributed to their smaller particle sizes and less agglomerations， while the introduction of water and boiling for the sample B leading to the particle growth and serious agglomeration may account for its lowest photocatalytic activity.

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[9]TSUZUKI T， MCCORMICK P G. ZnO nanoparticles synthesised by mechanochemical processing [J]. Scripta Mater， 2001，44（89）：17311734.〖ZK）〗

[10]〖ZK（#〗REDMOND G， OKEEFFE A， BURGESS C， et al. Determination of the flatband potential of transparent nanocrystalline zinc oxide films [J]. J Phys Chem， 1993，97（42）：1108111086.

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[13]QIAN D， GERWARD L， JIANG J Z. Comment on “Catalysis and temperature dependence on the formation of ZnO nanoparticles and of zinc acetate derivatives prepared by the solgel route”[J]. J Phys Chem B， 2004，108（39）：1543415435.